1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device (LCD). Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving the quality of picture image.
2. Discussion of the Related Art
As a thin film transistor liquid crystal display device (TFT-LCD) has been widely used for portable televisions or notebook computers, an LCD having a large panel is in great demand. A conventional TFT-LCD, however, has a problem that a contrast ratio is changed with a direction of viewing-angle. Liquid crystal display devices such as a twisted nematic LCD having an optical compensator and a multi-domain LCD have been proposed to cope with this problem. Nevertheless, such LCDs are not capable of solving the problem in a variation of the contrast ratio and color shifting.
An in-plane switching mode LCD to realize a wide viewing angle has also been proposed in the JAPAN DISPLAY 92 Page 457, Japanese Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined Publication No. 7-225538, and ASIA DISPLAY 95 Page 707.
A conventional in-plane switching mode LCD will now be explained with reference to FIGS. 1 to 3.
Referring first to FIGS. 1 and 2, operation of the conventional LCD will be described as follows. Liquid crystal molecules 8 in a liquid crystal layer 12 are aligned to have a rubbing direction (θR) of 90°<θR<180° with respect to a longitudinal elongation direction(0°) of a gate bus line on a substrate as shown in FIG. 2. A polarization axis direction (θPL2) of a analyzer 10 attached on a second substrate 5 is parallel to the rubbing direction (θR) . A polarization axis direction(θPL1) of a polarizer 9 attached on the first substrate 1 is perpendicular to a polarization axis direction (θPL2) and electrode elongation directions (θEL) of a data electrode 2 and a common electrode 3 are θEL=90° with respect to the longitudinal elongation direction of the gate bus line. Thus, when a voltage is not applied to a data electrode 2 and a common electrode 3 as shown in FIG. 1A, the liquid crystal molecules 8 are aligned with a slightly tilted direction relative to the elongation direction(θEL) of the data and common electrodes along with the rubbing direction(θR) in the substrate. The elongation direction(θEL) of the electrodes is perpendicular to the longitudinal direction of the gate bus line. Conversely, when a voltage having a horizontal electric field parallel to the longitudinal direction of the gate bus line is applied to the liquid crystal layer 12 as shown in FIG. 1E, the liquid crystal molecules 8 near the first substrate are rotated and a transmittance of the liquid crystal layer 12 is changed by a birefringence. A retardation value (Δnd) of the liquid crystal layer 12 is about λ/2 (for example, And would be approximately 0.21–0.36 μm, where λ is a wavelength of an incident light). For example, when the liquid crystal rotation angle is about 45 degree, the transmittance is maximum so that a screen of the LCD becomes a black mode.
FIG. 3A is a plane view of the conventional in-plane switching mode liquid crystal display device and FIG. 3B is a cross-sectional view taken along the line A-A′ in FIG. 3A. The liquid crystal display device is protected by a metal frame 22 excluding a representing unit 21 of a liquid crystal panel 32. A gate driving circuit 23, a data driving circuit 24, and a back light housing 25 including a back light 31 are mounted on the metal frame 22. In the representing unit 21, an exposure plate 75 (shown in FIG. 3B) having a light diffusion plate, polarizer 63, first and second substrates 27 and 26 constituting the liquid crystal panel 32, and an analyzer 64 are disposed on the second substrate 26. Further, a light compensator (not shown) may be disposed between the polarizer 63 and the first substrate 27 or between the second substrate 26 and the analyzer 64 to improve the contrast ratio.
Generally, in the conventional TFT-LCD, the TFT is formed in the first substrate 27 as a switching device and the color filter is formed on the second substrate 26. However, a diode may be used as a switching device in a diode LCD and a simple matrix LCD. Alternatively, when the TFT is formed on the second substrate, the color filter is formed onto the first substrate. Further, a mono-chromiumatic LCD may also be used without the color filter.
However, the conventional in-plane switching mode liquid crystal display device has a problem of the color shifting with the change of viewing angle direction. As shown in FIGS. 1C to 1D, when a horizontal electric field is applied to the electrodes 2, 3, the liquid crystal molecules 8 nearby the first substrate 1 are aligned parallel to the longitudinal direction of the gate bus line, whereas the liquid crystal molecules 8 nearby the second substrate 5 are aligned with an angle of 90°–180° relative to the longitudinal direction of the gate bus line. The liquid crystal molecules 8 are thus twisted. Therefore, color shifting is caused in either blue or yellow in a X or Y viewing angle direction, respectively. This color shifting mainly deteriorates the quality of the picture image.